It is a current trend to distribute content signals from broadcast stations and then play the content signals with an electronic machine, such as a smartphone, a personal computer, and a TV set. The content signals are encoded and distributed so as to protect distribution-related interests of a content distributor engaged in a paid broadcast service. In this regard, only those users which entered into a contract with the content distributor can decode the content signals and thereby watch the contents.
In the situation where content signals are encoded and distributed, typical measures taken to increase subscribers involve allowing viewers who have not entered into a contract with the content distributor to watch the contents for a short period of time, that is, see previews of the contents. To effectuate a preview, it is necessary to output a key which decodes encoded content signals for a specified period of preview time.